The present invention concerns a drained cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based molten electrolyte such as cryolite, having means to improve the distribution of dissolved alumina to enable a uniform electrolysis of alumina.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
A major drawback of conventional cells is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD), also called inter-electrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm to avoid short circuiting between the aluminium cathode and the anode or re-oxidation of the metal by contact with the CO2 gas formed at the anode surface.
Drained cell designs have been proposed to avoid the problems of conventional cells, by replacing the pool with a thin layer of aluminium which is drained down the surface of the cathode, enabling the Anode-Cathode Distance to be significantly reduced.
U.S. Pat. No. 4,560,488 (Sane/Wheeler/Kuivila) proposed a drained cathode arrangement in which the surface of a carbon cathode block was covered with a sheath that maintained stagnant aluminium on its surface in order to reduce wear. In this design, the cathode block stands on the cell bottom.
U.S. Pat. No. 3,400,061 (Lewis/Altos/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Stephen) disclose aluminium electrowinning cells with sloped drained cathodes arranged with the cathodes and facing anode surfaces sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the centre of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
An improvement described in U.S. Pat. No. 5,472,578 (de Nora) consisted in using grid-like bodies which could form a drained cathode surface and simultaneously restrain movement in the aluminium pool.
In. U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and re-immersion of the assembly during operation, such assembly also operating with a drained cathode.
U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved using a tubular anode with suitable openings.
Of course, the active surface of the cathode and of the anode should be at a slope to facilitate the escape of the bubbles of the released gas. Moreover, to have a cathode at a slope and obtain an efficient operation of the cell would be possible only if the surface of the cathode were aluminium-wettable so that the production of aluminium ions would take place on a film of aluminium.
Only recently has it become possible to coat carbon cathodes with a slurry which adheres to the carbon and becomes aluminium-wettable and very hard when the temperature reaches 700-800xc2x0 C. or even 950-1000xc2x0 C., as disclosed in U.S. Pat. No. 5,316,718 (Sekhar/de Nora) and U.S. Pat. No. 5,651,874 (de Nora/Sekar). These patents proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. These publications included a number of novel drained cathode configurations, for example including designs where a cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom. Further design modifications in the cell construction could lead to obtaining more of the potential advantages of these coatings.
European Patent Application No. 0 393 816 (Stedman) describes another design for a drained cathode cell intended to improve the bubble evacuation. However, the manufacture of the electrodes is difficult since their active surfaces slope along two orthogonal directions of the cell at the same time. Additionally, such a drained cathode configuration cannot ensure optimal distribution of the dissolved alumina.
U.S. Pat. No. 5,683,559 (de Nora) proposed a new cathode design for a drained cathode, where grooves or recesses were incorporated in the surface of blocks forming the cathode surface in order to channel the drained product aluminium. A specific embodiment provides an enhanced anode and drained cathode geometry where aluminium is produced between V-shaped anodes and cathodes and collected in recessed grooves. The V-shaped geometry of the anodes enables on the one hand a good bubble evacuation from underneath the anodes as described in the prior art, and on the other hand it enables the drainage of produced aluminium from cathode surfaces into recessed grooves located at the bottom of the V-shapes.
Whereas conventional cells having an aluminium pool motion require a greater Anode-Cathode Distance to prevent short-circuits between the electrodes, such pools provide sufficient motion in the electrolytic bath to distribute the dissolved alumina over the cathode. Conversely, drained cells have a reduced Anode-Cathode distance but do not have an aluminium pool motion that stirs and distributes alumina-rich electrolyte between the electrodes.
Because drained cells lack stirring means to distribute alumina-rich electrolyte in the Inter-Electrode Gap, areas of the cathodes which are close to the feeding point of alumina into the electrolyte contain greater amounts of alumina than remote areas.
Most of the alumina is electrolysed on the parts of the cathodes close to the dissolution point, whereas remote areas of the cathodes are poorly fed with alumina. This is due to the gradual depletion of the alumina concentration in the electrolyte while the electrolyte is moving between the electrodes where its electrolysis takes place. Consequently, such a gradient of dissolved-alumina concentration over the cathode of a drained cell can cause a non-uniform use of the active surfaces of the cathodes and therefore a non-uniform consumption of the electrodes while increasing the risk of a local anode effect due to a locally insufficient electrolysis of alumina.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggests a design improving the distribution of the dissolved alumina over the whole active surface of a drained cathode configuration.
It is therefore an object of the invention to provide a drained cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based melt such as cryolite, designed to ensure an enhanced distribution of alumina dissolved in electrolyte between the active sloping surfaces of the electrodes.
Another object of the invention is to provide a regular flow of the electrolyte containing CO2 gas towards the gap between the anodes and the subsequent return of electrolyte to the bottom at the lowest point of the anode surface where the alumina-rich electrolyte is formed.
The invention in particular relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte. Such cell comprises:
a) a cathode cell bottom comprising at least one sloped active cathode surface, and at least one recessed groove or channel below the bottom of the cathode active surface and extending therealong, the active cathode surface forming a drained cathode on which a layer of molten aluminium is produced and continuously drained into the recessed groove or channel;
b) at least one anode having sloped active anode surface facing the active cathode surface; and
When such cell is in use an electrolyte circulation is at least partly driven by gas released during the electrolysis between the sloped anode and cathode active surface.
The cell is characterized in that the means for feeding alumina are arranged such that alumina-rich electrolyte is fed into the or each recessed groove or channel which is arranged for the fed alumina-rich electrolyte to circulate longitudinally therein along substantially its entire length aboe the drained layer of aluminium. The recessed groove or channel further forms means for supplying the alumina-rich electrolyte to the bottom part of the or each active cathode surface under the effect of the electrolyte circulation produced by gas release.
In contrast to the prior art, the alumina enriched electrolyte is distributed over substantially the whole bottom end of the sloped active surface of the cathode.
The purpose of this invention is to supply the whole bottom part of the sloped cathode with alumina-rich electrolyte. To achieve this, the recessed groove or channel provides a sufficient flow of alumina-rich electrolyte to the active surfaces of the electrodes and additionally protects the supplied alumina-rich electrolyte from being electrolysed and depleted before it reaches the active surfaces where it is then electrolysed.
The recessed grooves or channels may be of any shape providing therein a sufficient electrolysis-free area for the required flow of alumina-rich electrolyte to the active surfaces of the electrodes. They may for instance be of constant section having a horizontal bottom, and therefore provide the active surfaces of the cathode bottom with a uniform flow of electrolyte from the recessed grooves or channels along the whole length thereof.
In order to enable an optimal draining of the product aluminium, the bottom of the recessed grooves or channels is preferably sloped.
Combining the two criteria described hereabove, a preferred geometry for each recessed groove or channel is a sloping bottom and a constant cross-sectional area along its length.
The above mentioned U.S. Pat. No. 5,683,559 (de Nora) describes in one embodiment a similar cathode bottom having sloped active surfaces and further provided with an aluminium collecting recessed groove along the bottom of the V-shaped surfaces of the cathode bottom and extending below the bottom of the sloped cathode surfaces. In contrast, the recessed grooves or channels of this invention must be drained or at least contain so little aluminium as to leave enough space above the level of the collected aluminium to allow a sufficient electrolyte circulation atop the collected drained aluminium within the recessed grooves or channels. Furthermore, such a recessed groove or channel provides an electrolysis-free electrolyte circulation wherein the supplied alumina-rich electrolyte is protected from the electrical current passing from the anodes to the cathode bottom.
In order to facilitate aluminium collection from the cell, cross-channels to which the recessed grooves or channels lead may be provided in the cell bottom. Such cross-channels are preferably located at the same level or below the level of the recessed grooves or channels to ensure an optimal evacuation of the product aluminium into the cross-channels and prevent the formation of thick layers of aluminium in the recessed grooves or channels. When the bottom of the recessed grooves or channels is sloping, such cross-channels are to be located at the lower end of said sloping bottoms. The bottom of the cross-channels is preferably sloping to facilitate aluminium evacuation.
Furthermore, the junctions between the cross-channels and the recessed grooves or channels can be advantageously used to locate alumina feeding points. However, it is not necessary to have alumina fed directly in front of the end opening of the recessed grooves or channels. Alumina can be fed anywhere where it is not subjected to immediate electrolysis but from where the alumina-rich electrolyte can reach the recessed grooves or channels before being exposed to the electrolysing electrical current.
The sloped active surfaces of the electrodes may be arranged freely provided the following conditions are met. Firstly, the sloping active surfaces should be so designed as to allow the produced gas accumulated in the form of bubbles under the anode active surfaces facing the cathode bottom to move freely along the anode bottom towards the surface and escape from there.
Additionally, in order to prevent over-depletion of the alumina-rich electrolyte during its electrolysis between the electrodes before it reaches the end of the active surfaces moved by the escaping gas bubbles, the length to be covered by the electrolyte between the electrodes should be reasonably short. This also offers the advantage of preventing the accumulation of gas into large bubbles.
For ease of manufacturing the cell, the sloping active cathode surfaces preferably form a series of juxtaposed V-shapes.
The cathode bottom of a cell according to the invention can be made of blocks having active sloped cathode surfaces, a bottom surface, a front surface, a back surface and two lateral surfaces. Such blocks may, for instance, comprise two V-shaped sloping active cathode surfaces and a recessed groove or channel below the bottom of the cathode active surfaces and extending therealong. Another possible design is a block comprising two roof-shaped sloping active cathode surfaces, each surface provided with a cut-out or a bevel below the bottom of the cathode active surfaces and extending therealong, so that a recessed groove or channel is formed between two laterally juxtaposed blocks. Alternatively this roof-shaped block can be obtained from the lateral juxtaposition of two part-blocks, each provided with only one sloping active surface and one cut-out or a bevel.
Such cathode blocks are advantageously provided with a grove or like recess in their bottom and extending therealong for receiving a steel or other conductive bar for the delivery of current. The groove is generally parallel to the active and lateral surfaces of the cathode block.
Normally the cathode blocks are made of carbon or carbonaceous material such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Pat. No. 5,413,689 (de Nora/Sekhar), prebaked carbon blocks assembled together on the shell, or graphite blocks, plates or tiles.
It is also possible for the cathode to be made mainly of an electrically-conductive carbon-free material, of a composite material made of an electrically-conductive material and an electrically non-conductive material, or of an electrically non-conductive material.
Carbon-free materials can be alumina, cryolite, or other refractory oxides, nitrides, carbides or combinations thereof. Carbon-free conductive materials is preferably chosen among Groups IIA, IIB, IIIA, IIIB, IVB, VB and the Lanthanide series, in particular aluminium, titanium, zinc, magnesium, niobium, yttrium or cerium, and alloys and intermetallic compounds thereof.
The composite material""s metal preferably has a melting point from 650xc2x0 C. to 970xc2x0 C.
The composite material is advantageously a mass made of alumina and aluminium or an aluminium alloy, see U.S. Pat. No. 4,650,552 (de Nora/Gauger/Fresnel/Adorian/ Duruz), or a mass made of alumina, titanium diboride and aluminium or an aluminium alloy.
The composite material can also be obtained by micropyretic reaction such as that utilising, as reactants, TiO2, B2O3 and Al.
The cathode can also be made of a combination of at least two materials from: at least one carbonaceous material as mentioned above; at least one electrically conductive non-carbon material; and at least one composite material of an electrically conductive material and an electrically non-conductive material, as mentioned above.
In any case a cell according to the invention is preferably provided with dimensionally stable anodes and cathodes. The anodes may for instance be made of non-carbon and substantially non-consumable material.
Advantageously the cathode surface is coated with an aluminium-wettable refractory material, such as a refractory hard metal boride. Particulate refractory hard metal boride may for instance be included in a colloidal carrier and then applied to the cathode surface, i.e. according to the teaching of the aforesaid U.S. Pat. No. 5,651,874 (de Nora/Sekhar).
The anodes of the electrolytic cell can be made of carbon-free material. In any case the anodes are preferably made of substantially non-consumable material.
The invention also relates to a method of electrowinning aluminium in a cell as described above.
The method is characterized in that feeding and dissolution of alumina in the electrolyte is followed by feeding the alumina-rich electrolyte into the or each recessed groove or channel and circulating alumina-rich electrolyte longitudinally in the or each recessed groove or channel aling substantially the entire length of the recessed groove or channel above the drained layer of aluminium. The alumina-rich electrolyte from the recessed groove or channel is then supplied to the bottom part of each active cathode surface under the effect of the electrolyte circulation produced by gas release from where it is distributed over the whole active cathode surface where it is electrolysed.
Alumina-rich electrolyte can be fed in different types of recessed grooves to provide dissolved alumina to the bottom part of the sloped surfaces. For instance, the electrolyte can be fed in at least one recessed groove or channel having a horizontal bottom, a sloped bottom or a bottom having a constant cross-sectional area along its length among many other possible shapes.
Aluminium produced on the active surfaces of the cathodes and drained into the recessed grooves or channels can be advantageously evacuated in at least one cross-channel preferably collecting aluminium from a plurality
Furthermore, fresh alumina can be fed at the junctions between the recessed grooves or channels and the cross-channels. Thus, alumina is dissolved closely to the recessed electrolyte supply grooves or channels.
As stated earlier, aluminium is preferably produced on sloping active cathode surfaces forming a series of juxtaposed V-shapes for ease of manufacturing the cathode cell bottom.
The electrolytic cell of the invention can either be obtained from a used conventional cell which is converted to the invention or a new cell specially designed for the purpose of the invention. In any case the manufacturing of the cell usually comprises providing channels, grooves, bevels, sloping sections or cut-outs in the top surface of the cathode bottom of the cell before or after assembly of the components of the cell. The channels or grooves or sloping sections can be machined in the top surfaces of the cathode bottom of the cell.